You might not need to go higher in watts at all if you like your flavour

lots of folks stick in the 8 to 12 watt range, even though high watts is fashionable these days.
The nautilus is a really good tank so i would be hesitant to change it if you are happy with your vape. Many folks would recommend the subtank mini from kanger, i have it but i still think the nautilus has a better flavour just not as much vapour.
Most tanks and batteries/mods have the standard 510 style connection so they will work. The difference mostly comes into how much watts your coil and tank combo want compared to what the battery/mod can produce.
Are you happy with the taste of what yoy have?
This is the lowest price i have found for genuine coils, good vendor, many happy customers here..
Aspire Nautilus BVC Coils Organic Cotton
Very little difference between the 1.6 and 1.8 i would just go with the 1.8 for a while unless/until you might want a bit more 'cloudz'